Thermal type flow rate measuring apparatus

ABSTRACT

A flow rate sensor has a problem that a resistance value of a heat generating resistor itself varies and sensor characteristics are changed during use of the sensor for a long term. Also, the temperature of the heat generating resistor must be adjusted on a circuit substrate with a resistance constituting one side of a fixed temperature difference control circuit, and this has been one of factors pushing up the production cost. All resistances used for fixed temperature difference control are formed on the same substrate as temperature sensitive resistors of the same material. This enables all the resistances for the fixed temperature difference control to be exposed to the same environmental conditions. Hence, even when the resistances change over time, the changes over time occur substantially at the same tendency. Since the resistances for the fixed temperature difference control change over time essentially at the same rate, a resulting output error is very small.

This is a continuation application of U.S. Ser. No. 10/449,117, filedJun. 2, 2003 now U.S. Pat. No. 6,925,866.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a apparatus for detecting a flow rate,and more particularly to, for example, a flow sensor for use in aninternal combustion engine or a flow sensor for use in a fuel batterysystem.

2. Description of the Related Art

Hitherto, as air flow sensors disposed in intake air passages ofinternal combustion engines of automobiles, etc. for measuring theamounts of intake air, thermal ones have been primarily employed becauseof a capability of directly detecting air mass flow rates. Recently, airflow sensors manufactured by semiconductor micromachining techniques, inparticular, have received attention because they have a high-speedresponse and are able to detect a back flow by utilizing the high-speedresponse.

One example of the conventional technique regarding such a thermal typeair flow sensor using a semiconductor substrate is disclosed in, e.g.,Japanese Publication of Examined Patent Application No. 6-63801. In therelated art, an electric current is supplied to a heater resistancearranged between an upstream-side temperature sensor and adownstream-side temperature sensor for generating heat, and a flow ratesignal is obtained depending on a difference in output signal betweenthe upstream-side temperature sensor and the downstream-side temperaturesensor.

SUMMARY OF THE INVENTION

The related art, however, has a problem that the resistance value of aheat generating resistor itself is changed with heating of the heatgenerating resistor, which is formed in a thin wall portion.

In view of the above problem, it is proposed to provide a compensatingmeans in a control circuit so that flow rate characteristics will notchange even with changes of the resistance value, as described in theabove-cited Japanese Publication of Examined Patent Application No.6-63801.

However, such a proposal raises problems that the circuit structure iscomplicated and the sensor structure is also complicated. Further, noconsiderations are paid to time-dependent changes of parts mounted on acircuit board.

Moreover, for control of the temperature of the heat generatingresistance, a resistance constituting one side of a fixed temperaturedifference bridge must be adjusted on the circuit board. This necessityhas been one of factors pushing up the production cost.

Furthermore, a fixed resistance used in the fixed temperature differencebridge, which is formed on the circuit board, in fact has a slightresistance temperature coefficient, and the heating temperature of theheat generating resistor varies when the ambient temperature is changedwith a variation in the resistance temperature coefficient, thusresulting in a variation in temperature characteristics.

Also, the fixed resistance is itself changed over time, which causescharacteristic changes.

In addition, recently, a demand for protection against electromagneticinterference has enlarged and the necessity of improving durabilityagainst electromagnetic interference has increased.

In the view of the above-mentioned problems in the related art, it is anobject of the present invention to provide a thermal type flow measuringdevice, which is inexpensive and has high reliability.

The above object is achieved with the features of the present inventionset forth in claims.

More specifically, by forming resistors, which constitute a fixedtemperature difference control circuit, as temperature sensitiveresistors, the resistors can be caused to change their resistances atthe same tendency if the resistance changes should-occur over time.

When resistance values of the resistors for the fixed temperaturedifference control change over time at the same rate, a resulting outputerror is very small and therefore reliability can be improved. As thefixed temperature difference control circuit, a bridge circuit ispreferably employed because it is of a relatively simple structure andhas superior advantages.

Also, with the fixed temperature difference control circuit formed usingidentical temperature sensitive resistors, these components can beformed at a time in the same manufacturing process and the abovestructure is more cost effective. In particular, a variation inresistance temperature coefficients of the temperature sensitiveresistors can be reduced and therefore a variation in temperaturecharacteristics can be reduced correspondingly.

Since the heat generating resistor is also formed in the fixedtemperature difference control circuit, the circuit configuration issimplified, which is advantageous in reducing the apparatus size.

Since resistances for the fixed temperature difference control,including those ones which have been mounted on a separate circuit boardin the past, are all formed on one substrate, lengths of wiring forconnection between the resistances can be minimized. Hence, a wiringsection is less likely to serve as an antenna and is very endurableagainst electromagnetic interference.

When the thermal type flow measuring apparatus is applied to anautomobile, it is mounted in an engine room. In such a case, a regionnear a sensor section, which is less subjected to heat radiation from anengine and is exposed to intake air, is more advantageous from theviewpoint of temperature environmental conditions. By forming theresistances for the fixed temperature difference control on onesubstrate together with the heat generating resistor and arranging thesubstrate to be exposed to a fluid, the present invention isadvantageously implemented for the purpose of compensation of resistancechanges over time.

Further, by forming the resistances for the fixed temperature differencecontrol in the same manufacturing process, the resistance ratio can bealways controlled constant in spite of absolute values of theresistances having a large variation. With the resistance ratio heldconstant, the fixed temperature difference control circuit can bemanufactured without adjustments to be substantially free from avariation in the heating temperature of the heat generating resistor.

As a result, operations, such as laser trimming, for adjusting theheating temperature of the heat generating resistor, which have beenrequired in the past, are no longer required, and manufacturing stepscan be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a wiring pattern diagram of a thermal type flow sensoraccording to one embodiment of the present invention;

FIG. 2 is a circuit diagram including the thermal type flow sensor shownin FIG. 1;

FIG. 3 is a sectional view showing a thermal type flow measuringapparatus including the thermal type flow sensor shown in FIG. 1;

FIG. 4 is a graph for explaining a variation in temperaturecharacteristics of the thermal type flow measuring apparatus accordingto the present invention;

FIG. 5 is a graph for explaining one example of resistance changes of aheat generating resistor over time;

FIG. 6 is a wiring pattern diagram of a thermal type flow sensoraccording to another embodiment of the present invention;

FIG. 7 is a circuit diagram including the thermal type flow sensor shownin FIG. 6;

FIG. 8 is a wiring pattern diagram of a thermal type flow sensoraccording to still another embodiment of the present invention;

FIG. 9 is a circuit diagram including the thermal type flow sensor shownin FIG. 8;

FIG. 10 is a wiring pattern diagram of a thermal type flow sensoraccording to still another embodiment of the present invention;

FIG. 11 is a wiring pattern diagram of a thermal type flow sensoraccording to still another embodiment of the present invention;

FIG. 12 is a circuit diagram including the thermal type flow sensorshown in FIG. 11;

FIG. 13 is a sectional view showing a thermal type flow measuringapparatus having the thermal type flow sensor shown according to thepresent invention;

FIG. 14 is a partial enlarged view of a thermal type flow sensoraccording to still another embodiment of the present invention;

FIG. 15 is a graph of a temperature distribution of a heat generatingresistor for explaining an advantage of the thermal type flow sensoraccording to the present invention shown in FIG. 14;

FIG. 16 is a graph of a temperature distribution of a heat generatingresistor for explaining an advantage of the thermal type flow sensoraccording to the present invention shown in FIG. 14;

FIG. 17 is a wiring pattern diagram of a thermal type flow sensoraccording to still another embodiment of the present invention;

FIG. 18 is a circuit diagram including the thermal type flow sensorshown in FIG. 17;

FIG. 19 is a wiring pattern diagram of a thermal type flow sensoraccording to still another embodiment of the present invention;

FIG. 20 is a circuit diagram including the thermal type flow sensor;

FIG. 21 is a system diagram of an internal combustion engine to whichthe present invention is applied;

FIG. 22 is a circuit diagram including a thermal type flow sensor of theprior art;

FIG. 23 is a table for explaining an advantage of the thermal type flowmeasuring apparatus according to the present invention; and

FIG. 24 is a table for explaining an advantage of the thermal type flowmeasuring apparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the drawings.

FIG. 1 is a diagram showing a structure and a wiring pattern of athermal type flow sensor 26 used in a thermal type flow measuringapparatus 1 according to one embodiment of the present invention. FIG. 2is a circuit diagram including the thermal type flow sensor 26 shown inFIG. 1. FIG. 3 is a sectional view showing a state in which the thermaltype flow measuring apparatus 1 is actually mounted in an intake pipe ofan internal combustion engine. A description is made of one embodimentof the present invention with reference to FIGS. 1, 2 and 3.

As shown in FIG. 1, the thermal type flow sensor 26 is of a structurethat a thin wall portion 7 is formed in a substrate 2 of asemiconductor, etc., and a heat generating resistor 10, an upstream-sidetemperature sensor 30 and a downstream-side temperature sensor 31 arearranged in the thin wall portion 7 to be thermally insulated from thesubstrate 2. Then, temperature sensitive resistors 11, 12 and 13 areformed of the same material as that of the heat generating resistor 10.Each of these resistors 10 to 13 has an electrode 51 made of, e.g.,aluminum for electrical connection to the outside. The substrate 2 of asemiconductor, etc. is formed of, e.g., silicon. The thin wall portion 7is obtained by forming a cavity in the substrate 2 from its rear sidewith anisotropic etching using an etchant such as potassium hydroxide.In the thermal type flow sensor 26 thus constructed, the substrate 2 hasdimensions of about 2.5 mm×6 mm×0.3 mm, and the thin wall portion 7 hasdimensions of about 0.5 mm×1 mm and a thickness of about 0.0015 mm. Thetemperature sensitive resistors are each generally made of a resistorformed by doping impurities in poly- or single-crystalline silicon, ormade of platinum, gold, copper, aluminum, chromium, nickel, tungsten,Permalloy (FeNi), titanium, etc.

As shown in FIG. 2, the heat generating resistor 10 constitutes a bridgecircuit in cooperation with the temperature sensitive resistors 11, 12and 13. The heating temperature of the heat generating resistor 10 isdecided based on resistance values of the temperature sensitiveresistors 11, 12 and 13. The heating of the heat generating resistor 10is controlled so as to hold it substantially at a fixed temperaturedifference ΔT relative to the ambient temperature under feedback controlusing a differential amplifier 41, a transistor 45, etc., and if theambient temperature is the same, the heating control is performed so asto hold the heat generating resistor 10 substantially at a fixedtemperature regardless of a flow rate. More specifically, in the case ofthe heat generating resistor 10 which is controlled to be held at ΔT of150° C., for example, the heating temperature is about 170° C. when theambient temperature is 20° C., and it is about 250° C. when the ambienttemperature is 100° C.

The heating temperature of the heat generating resistor 10 is controlledto be held substantially the same in spite of changes of the flow rate.Hereinafter, that type of control method is referred to simply as afixed temperature difference control method, and a circuit for realizingsuch a control method is referred to as a fixed temperature differencebridge or a fixed temperature difference control circuit in thisspecification.

An output 44 of the thermal type flow measuring apparatus 1 can beobtained by a temperature sensor bridge made up of temperature sensors30, 31, which are formed respectively upstream and downstream of theheat generating resistor 10, and fixed resistances 61, 62. Statedotherwise, in a state of no air flow, because the upstream-sidetemperature sensor 30 and the downstream-side temperature sensor 31 aresubjected to the same temperature, the output 44 is zero when theresistance values of the upstream-side and downstream-side temperaturesensors 30, 31 are equal to each other and the resistance values of thefixed resistances 61, 62 are equal to each other.

When there occurs a flow and a fluid flow 6 is caused in a directionfrom the upstream to downstream side, for example, as shown in FIG. 1,the upstream-side temperature sensor 30 is cooled while the temperatureof the downstream-side temperature sensor 31 conversely rises uponreceiving heat generated by the heat generating resistor 10. This causesa difference between the resistance values of the upstream-side anddownstream-side temperature sensors 30, 31, whereby the output 44indicative of the flow rate is produced. Likewise, when there occurs afluid flow from the downstream to upstream side, the output 44 ischanged in a direction reversal to that in the above case. It is hencepossible to detect the flow rate of a fluid including a back flow.

The thermal type flow measuring apparatus 1 of the present invention isused, for example, to detect the flow rate of intake air in an internalcombustion engine and to supply fuel in match with the detected air flowrate. To that end, the thermal type flow measuring apparatus 1 isdisposed between an air cleaner 102 and a throttle body 109 (see FIG.21) in an engine room, and includes a circuit board 4, a housing 3, etc.as shown in FIG. 3. A sub-passage 52 is formed in a main passage 5, andthe thermal type flow sensor 26 is disposed in the sub-passage 52.

At the startup of engine operation, the temperature is equal to that ofopen air and the temperature of intake air is in the range of −30° C. to40° C. After the start of warming-up, however, the temperature at thesurface of the main passage 5 rises to about 125° C. at maximum and thetemperature of intake air rises to about 100° C. at maximum because ofan effect of heat generated from the engine. For that reason, thethermal type flow measuring apparatus 1 is required to cause no outputerrors over a wide temperature range of −30° C. to 100° C. or 125° C. Ina conventional thermal type flow measuring apparatus (corresponding to1), a part of resistances of a fixed temperature difference bridge forcontrolling a heat generating resistor (corresponding to 10) to be keptat a fixed temperature difference is formed as a fixed resistance on acircuit board (corresponding to 4). In spite of being the fixedresistance, however, its resistance temperature coefficient is notperfectly zero, and even the fixed resistance generally has a resistancetemperature coefficient of about 0±50 ppm. Such a variation in theresistance temperature coefficient makes the bridge out of balance upona change of the ambient temperature and is one of factors causing avariation in temperature characteristics.

Also, it is assumed in cold districts that an engine is warmed up in agarage and an automobile starts traveling after the warming-up. In thiscase, immediately after the start of traveling, there occurs a conditionthat the surface of the main passage 5 is heated with heat generated bythe engine, while intake air remains cooled. In such a transientcondition, the thermal type flow sensor 26 exposed to the intake air isat a cold temperature, while the circuit board 4 is at an increasedtemperature. In the conventional case, there occurs no problem if theresistance temperature coefficients of the fixed resistances of thefixed temperature difference bridge mounted on the circuit board(corresponding to 4) are perfectly zero. However, if the resistancetemperature coefficient has a variation of about 0±50 ppm as mentionedabove, the bridge balance is lost and the sensor output (correspondingto 44) is varied.

Further, because the resistance values of the heat generating resistor10 and the resistances of the fixed temperature difference bridgeinevitably have manufacturing variations, the heating temperature of theheat generating resistor 10 undergoes a very large variation if noadjustment is performed. In the conventional thermal type flow measuringapparatus (corresponding to 1), therefore, the resistances of the fixedtemperature difference bridge must be individually adjusted on thecircuit board (corresponding to 4) by trimming using a laser, forexample, and this necessity is another factor pushing up the cost.

Moreover, because the heating temperature of the heat generatingresistor 10 is set to a level, e.g., about 100° C. to 200° C. higherthan the ambient temperature, the actual temperature of the heatgenerating resistor 10 is about 200° C. to 300° C. at maximum when thetemperature of intake air is 100° C.

During use for a long period of time, therefore, the heat generatingresistor 10 gradually deteriorates and its resistance value changescorrespondingly. Also, the temperature of the circuit board 4 is raisedto about 125° C. substantially equal to the surface temperature of themain passage 5 because the circuit board 4 is subjected to not only athermal effect similarly caused by the heating temperature of the heatgenerating resistor 10 mentioned above, but also a thermal effect causedby heat generating parts, such as a transistor, mounted on the surfaceof the circuit board 4. When the fixed resistances of the fixedtemperature difference bridge are formed by printing, for example, as inthe related art, the fixed resistances also gradually deteriorate andtheir resistance values change correspondingly. This leads to a problemthat flow rate characteristics are changed with resistance changes.

In addition, although metallic materials, such as aluminum, were used asthe main passage 5 in the past, resin materials have been recently usedin many cases for weight reduction. On the other hand, there is atendency toward a sharper demand for protection against electromagneticinterference, and an improvement in durability against electromagneticinterference is required.

A description is now made of superior points resulting from forming theresistances of the fixed temperature difference bridge as thetemperature sensitive resistors on the same substrate as in the presentinvention.

First, the fact that the thermal type flow measuring apparatus accordingto the present invention, in which the fixed temperature differencebridge is entirely made up of the temperature sensitive resistors,properly operates is described below in comparison with the conventionalone.

In the circuit diagram of FIG. 2, the fixed temperature differencebridge is constituted as a section made up of the heat generatingresistor 10 and the temperature sensitive resistors 11, 12 and 13.Though not shown, a modification, in which the temperature sensitiveresistors 12, 13 of the fixed temperature difference bridge are formedas fixed resistors, corresponds to the conventional fixed temperaturedifference bridge.

Assuming that, in the fixed temperature difference bridge shown in FIG.2, the resistance value of the heat generating resistor 10 at 0° C. isRh0, the resistance value of the temperature sensitive resistor 11 at 0°C. is Ra0, the resistance value of the temperature sensitive resistor 12at 0° C. is Rb0, the resistance value of the temperature sensitiveresistor 13 at 0° C. is Rc0, and all the resistance temperaturecoefficients (° C.⁻¹) of these resistors have the same value α,respective resistance values of these resistors at an arbitrarytemperature T° C. can be expressed by the following formulae (1), (2),(3) and (4);Rh=Rh0×(1+α×T)  (1)Ra=Ra0×(1+α×T)  (2)Rb=Rb0×(1+α×T)  (3)Rc=Rc0×(1+α×T)  (4)wherein Rh, Ra, Rb and Rc are respective resistance values of theresistors at an arbitrary temperature T° C.

Also, since Rh (i.e., the heat generating resistor 10) is controlled tobe held at a level fixed temperature ΔT° C. higher than that of Ra, Rband Rc (i.e., the temperature sensitive resistors 11, 12 and 13), theformula (1) can be rewritten to the following formula (5);Rht=Rh0×(1+α×(T+ΔT))  (5)wherein Rht is a resistance value of the heat generating resistor 10resulting when the fixed temperature difference bridge is actuallyoperated.

On the other hand, the equilibrium condition of the fixed temperaturedifference bridge can be expressed by the following formula (6):Rht×Rc=Ra×Rb  (6)

By putting the formulae (2), (3), (4) and (5) in the formula (6), thefollowing formula (7) is obtained;

$\begin{matrix}{{{Rh}\; 0 \times \left( {1 + {\alpha \times \left( {T + {\Delta\; T}} \right)}} \right) \times {Rc}\; 0 \times \left( {1 + {\alpha \times T}} \right)} = {{Ra}\; 0 \times \left( {1 + {\alpha \times T}} \right) \times {Rb}\; 0 \times \left( {1 + {\alpha \times T}} \right)}} & (7)\end{matrix}$

Assuming here the condition that Rb and Rc are at the same temperature,the formula (7) can be rewritten to the following formula (8):

$\begin{matrix}{{{Rh}\; 0 \times \left( {1 + {\alpha \times \left( {T + {\Delta\; T}} \right)}} \right)} = {{Ra}\; 0 \times \left( {1 + {\alpha \times T}} \right) \times {Rb}\;{0/{Rc}}\; 0}} & (8)\end{matrix}$

This formula (8) is exactly the same as the equilibrium condition in thecase in which Rb and Rc are given by the fixed resistances, and impliesthat the fixed temperature difference control can be performed.

In other words, the inventors have paid attention to the fact that, byforming the fixed temperature difference bridge so as to provide Rb andRc to be held at the same temperature, all the resistances of the fixedtemperature difference bridge can be formed as the temperature sensitiveresistors.

According to the present invention, therefore, the fixed temperaturedifference bridge is of a structure in which the substrate 2 is made of,e.g., silicon having good thermal conductivity and the temperaturesensitive resistors 12, 13 (Rb, Rc) are formed on such a substrate, orin which the temperature sensitive resistors 12, 13 (Rb, Rc) arearranged close to each other as shown in FIG. 1, in order that thetemperature sensitive resistors 12, 13 (Rb, Rc) are held at the sametemperature.

Alternatively, as seen from the formula (6), the above discussion isalso likewise established when the ambient temperature is detected usingRb instead of Ra and the temperature sensitive resistors 11, 13 (Ra, Rc)are held at the same temperature.

By forming the fixed temperature difference bridge of the same materialin the same manufacturing process, the following advantages areobtained.

A variation in absolute ones of the resistance values is, thoughdepending on process conditions, about ±30%. However, when the resistorsare formed by patterning in the same manufacturing process, there occursno appreciable variation in a resistance ratio Rh/Rb of Rh to Rb and aresistance ratio Ra/Rc of Ra to Rc. Further, because all the resistancesare formed on the same substrate, there also occurs no appreciablevariation in their resistance temperature coefficients.

When the resistors are actually formed by patterning with thesemiconductor process and the resistance ratio of the resistors isactually measured, the values of Rh/Rb and Ra/Rc are each not largerthan ±0.07%. Also, it has been confirmed that a variation in absolutevalues of the resistance temperature coefficients is about ±1% amonglots, whereas the variation can be made substantially zero for the samesubstrate 2 within the same lot.

Consequently, as shown in Table 1 (FIG. 23), the application of thepresent invention ensures that a variation in the heating temperature ofthe heat generating resistor 10 can be held at about ±1° C. withoutadjustments. This variation implies a great improvement as compared withthe prior art in which a variation in the heating temperature is ±14°C., and represents a level that can be satisfactorily employed withoutadjustments.

With the application of the present invention, therefore, it is possibleto omit a step of adjusting the heating temperature of the heatgenerating resistor 10, and hence to cut down the production cost.Further, since the fixed resistances of the fixed temperature differencebridge are not formed on the circuit board 4, the size of the circuitboard 4 can be reduced.

Moreover, the structure resulting from the application of the presentinvention, in which all the temperature sensitive resistors are formedon the substrate 2 and are exposed to intake air, is advantageous fromthe thermal point of view because, as described above, the temperatureof the surface of the main passage 5 and the circuit board 4 rises toabout 125° C. at maximum, the temperature of intake air rises to a levelas high as about 100° C. As a result, changes of Rb and Rc over time canbe reduced.

In addition, even when Rb and Rc are changed over time, Rb and Rcexhibit similar changes because they undergo heat history under the sameenvironmental conditions. Thus, the balance of the fixed temperaturedifference bridge is not changed, whereby the heating temperaturedifference ΔT of the heat generating resistor 10 can be prevented fromchanging. As a result, it is possible to prevent changes in the output44 and to improve reliability.

Furthermore, with the application of the present invention, a variationin temperature characteristics is also reduced as seen from Table 1(FIG. 23) and Table 2 (FIG. 24). As shown in FIG. 24, a variation in theresistance temperature coefficients of the temperature sensitiveresistors is, e.g., 2000±20 ppm/° C. Also, a variation in the resistancetemperature coefficients within the same substrate 2 is substantiallyzero, i.e., about ±2 ppm/° C. With the application of the presentinvention, the heating temperature of the heat generating resistor 10 isabout 170° C.±2° C. when the ambient temperature is 20° C., and it isabout 247° C.±2° C. when the ambient temperature is changed to 80° C. Itis thus confirmed that a variation in the heating temperature of theheat generating resistor 10 is very small even with changes of theambient temperature.

On the other hand, in the case of the prior art, the resistancetemperature coefficients of the fixed resistances (corresponding to Rband Rc) formed on the circuit board (corresponding to 4) by, e.g.,printing are each basically 0 ppm/° C., but have a large variation ofabout ±50 ppm/° C. More specifically, in the case of the prior art, whenthe ambient temperature is 20° C., the heating temperature of the heatgenerating resistor 10 is about 170° C.±3° C. and a variation in theheating temperature can be held relatively small. However, when theambient temperature is changed to 80° C., the heating temperature of theheat generating resistor 10 is about 247° C.±8° C. and a variation inthe heating temperature is increased. This increased variation increasesa variation in temperature characteristics.

FIG. 4 shows a comparison of a variation in temperature characteristicsbetween the present invention and the prior art resulting when theambient temperature is changed from 20° C. to 80° C. A region denoted by(a) in FIG. 4 represents a variation in temperature characteristics inthe present invention, and a region denoted by (b) represents avariation in temperature characteristics in the prior art. As seen fromFIG. 4, with the application of the present invention, a variation intemperature characteristics can be greatly reduced.

Further, with the structure of the present invention in which the fixedtemperature difference bridge is entirely formed on the same substrate2, a variation in the resistance temperature coefficients of Rb, Rc canbe made very small and a temperature difference can be made less likelyto occur between Rb and Rc (temperature sensitive resistors 11 and 12)by, for example, arranging them close to each other. Accordingly, afluctuation of the output 44 can be prevented even in environmentalconditions in which there occurs a temperature difference between theintake air and the substrate.

Moreover, since the fixed temperature difference bridge is entirelyformed on the same substrate 2, lead lines for connection between thetemperature sensitive resistors can be made very short, which isadvantageous for protection against electromagnetic interference. Thisis because, in the prior-art structure in which the resistances of thefixed temperature difference bridge are formed on both the substrate(corresponding to 2) and the circuit board (corresponding to 4), longlead lines are required and serve as antennas receiving radio waves,whereas such a drawback is eliminated in the present invention.

As alternative means for obtaining similar advantages to those describedabove, it is conceivable, for example, to form Rh, Ra (i.e., the heatgenerating resistor 10 and the temperature sensitive resistor 11) on onesubstrate 2, to form Rb, Rc (i.e., the temperature sensitive resistors12, 13) on another substrate 2, and to combine the two substrates witheach other. This modification increases the number of steps required,but can provide substantially the same advantages as those describedabove, i.e., an omission of the step of adjusting the heatingtemperature and a smaller variation in temperature characteristics.Another advantage is from the thermal point of view in that Rb, Rc(i.e., the temperature sensitive resistors 12, 13) can be positionedaway from heat generating resistor 10.

When aiming at an improvement of reliability as a main object, asatisfactory effect can be obtained just by forming all the resistancesof the fixed temperature difference bridge as the temperature sensitiveresistors of the same material. The reason resides in that, by using thesame material, the temperature sensitive resistors change over time at arelatively matched tendency, and therefore the balance of the fixedtemperature difference bridge is not changed.

The upstream-side temperature sensor 30 and the downstream-sidetemperature sensor 31 can be formed of any suitable one of variousmaterials. As one example, those sensors may be formed of the samematerials as that of the heat generating resistor 10 and the temperaturesensitive resistors 11, 12 and 13. Using the same material isadvantageous in simplifying the manufacturing process because all theresistors can be formed with only one sequence of resistance formingprocesses.

Another embodiment of the present invention will be described below withreference to FIGS. 6 and 7.

FIG. 6 is a diagram showing a structure and a wiring pattern of athermal type flow sensor 26 according to this embodiment, and FIG. 7 isa circuit diagram including the thermal type flow sensor 26 shown inFIG. 6. A method of manufacturing the thermal type flow sensor 26 issimilar to that described above with reference to FIG. 1, and adescription thereof is omitted here.

As shown in FIG. 6, a thin wall portion 7 is formed nearly in a centralarea of a substrate 2, and a temperature sensitive resistor 24 and aheat generating resistor 10 are arranged in the thin wall portion 7close to each other. Upstream-side temperature sensors 30, 33 anddownstream-side temperature sensors 31, 32 are formed respectivelyupstream and downstream of the heat generating resistor 10. The heatgenerating resistor 10 and four temperature sensitive resistors 22, 23,24 and 25 are formed in the same manufacturing process using the samematerial. As a matter of course, it is more preferable forsimplification of the manufacturing process that the upstream-side anddownstream-side temperature sensors 30, 33, 31 and 32 be also formed inthe same manufacturing process.

As shown in FIG. 7, the temperature sensitive resistor 24 is connectedto the other temperature sensitive resistors 22, 23 and 25, all of whichare formed on the same substrate 2, thereby forming a bridge. When theheat generating resistor 10 is cooled by a fluid flow, the temperaturesensitive resistor 24 arranged close to the heat generating resistor 10is also cooled, whereupon the bridge balance is changed. The heatgenerating resistor 10 is controlled to be substantially at a fixedtemperature difference by controlling the change of the bridge balancein a feedback manner using a differential amplifier 41, a transistor 45,etc.

The present invention can also be applied to a fixed temperaturedifference control scheme employing the heat generating resistor 10 andthe four temperature sensitive resistors 22, 23, 24 and 25, which arearranged as described above, and that type of the fixed temperaturedifference control scheme is also involved within the concept of thefixed temperature difference control circuit according to the presentinvention. Though not explained in detail, the concept of the presentinvention is further applicable to conventionally known other types offixed temperature difference control circuits. In any case, at leastfour or more temperature sensitive resistors are required for the fixedtemperature difference control.

Note that the advantages of this embodiment are the same as thosedescribed above, and hence a description thereof is omitted here.

Still another embodiment of the present invention will be describedbelow with reference to FIGS. 8 and 9.

FIG. 8 is a diagram showing a structure and a wiring pattern of athermal type flow sensor 26 according to this embodiment, and FIG. 9 isa circuit diagram including the thermal type flow sensor 26 shown inFIG. 8. A method of manufacturing the thermal type flow sensor 26 issimilar to that described above with reference to FIG. 1, and adescription thereof is omitted here.

As shown in FIG. 8, a thin wall portion 7 is formed in a substrate 2,and two heat generating resistors 14, 18 are arranged in the thin wallportion 7 close to each other respectively on the upstream anddownstream sides with respect to a fluid flow. Then, three temperaturesensitive resistors 15, 16 and 17 are connected to the upstream-sideheat generating resistor 14, and three temperature sensitive resistors19, 20 and 21 are connected to the downstream-side heat generatingresistor 18. These resistors are all formed as temperature sensitiveresistors in the same manufacturing process using the same material. Theheat generating resistors 14, 18 are operated while constitutingrespective fixed temperature difference bridges independent from eachother as shown in FIG. 9. When there occurs a fluid flow 6, heatgenerated by the upstream-side heat generating resistor 14 is receivedby the downstream-side heat generating resistor 18 and therefore theamount of heat radiated from the downstream-side heat generatingresistor 18 is reduced. When there occurs a fluid flow in a reverseddirection, the amount of heat radiated from the upstream-side heatgenerating resistor 14 is reduced. By utilizing such a phenomenon, anoutput 47 representing the direction of the fluid flow and an output 44representing the flow rate can be obtained.

The present invention is particularly effective when applied to athermal type flow measuring apparatus 1 having the above-describedstructure.

More specifically, an error occurs unless a heating temperaturedifference ΔT between the two heat generating resistors 14 and 18behaves essentially in the same way when the ambient temperature ischanged or when the flow rate is changed. By forming all the resistancesof the two fixed temperature difference bridges as the temperaturesensitive resistors identical to each other as in the present invention,the heating temperature difference ΔT can be caused to always behaveessentially in the same way because the resistance ratios and theresistance temperature coefficients of the two fixed temperaturedifference bridges are held substantially equal to each other. Also, by,as shown in FIG. 8, arranging the temperature sensitive resistors 15, 19of the two fixed temperature difference bridges close to each other tohave the same temperature as far as possible and likewise arranging thetemperature sensitive resistors 16, 17, 20 and 21 close to each other tohave the same temperature as far as possible, the heating temperaturedifference ΔT between the two temperature sensitive resistors 14 and 18is caused to behave more exactly in the same manner. Consequently, themeasurement can be performed with higher accuracy. Other advantages ofthis embodiment are the same as those described above, and hence adescription thereof is omitted here.

Still another embodiment of the present invention will be describedbelow with reference to FIG. 10.

FIG. 10 is a diagram showing a structure and a wiring pattern of athermal type flow sensor 26 according to this embodiment. A ceramicsubstrate made of alumina and having a thickness of about 0.1 mm is usedas a substrate 2. On one surface of the substrate 2, after forming athin film of platinum, for example, by sputtering, a heat generatingresistor 10 and temperature sensitive resistors 11, 12 and 13 are formedon the thin film by patterning.

The heat generating resistor 10 and the temperature sensitive resistors11, 12 and 13 are connected so as to form a fixed temperature differencebridge. Then, as shown in FIG. 10, a slit 54 is formed between the heatgenerating resistor 10 and the temperature sensitive resistors 11, 12and 13 to prevent thermal conduction from the heat generating resistor10 to the temperature sensitive resistors 11, 12 and 13. The presentinvention can also be applied to the thermal type flow sensor 26 thusconstructed. Note that advantages of this embodiment are the same asthose described above, and hence a description thereof is omitted here.

Still another embodiment of the present invention will be describedbelow with reference to FIG. 11.

As shown in FIG. 11, a thin wall portion 7 is formed in a substrate 2,and the thin wall portion 7 is thermally insulated from the substrate 2.A heat generating resistor 10 is formed in the thin wall portion 7, andtemperature sensitive resistors 11, 12, 13, 13 a and 13 b are formed andelectrically connected to the heat generating resistor 10. Resistancevalues of the temperature sensitive resistors 13 a, 13 b are each setnot more than 10% of that of the temperature sensitive resistor 13. Onan assumption that the resistance value of the heat generating resistor10 under operation is Rht, the resistance value of the temperaturesensitive resistor 11 under operation is Ra, the resistance value of thetemperature sensitive resistor 12 under operation is Rb, the resistancevalue of the temperature sensitive resistor 13 under operation is Rc,the resistance value of the temperature sensitive resistor 13 a underoperation is Rca, the resistance value of the temperature sensitiveresistor 13 b under operation is Rcb, and all the resistance temperaturecoefficients of these resistors have the same value α, formation of abridge expressed by the following formula (9), for example, can beassumed:Rht×Rc=Ra×Rb  (9)

The heating temperature of the heat generating resistor 10 inevitablyhas a variation of about ±1° C. to ±2° C. as shown in FIGS. 23 and 24,and such a variation cannot be perfectly eliminated. In order to performthe flow measurement with higher accuracy, it is therefore morepreferable to change the bridge balance, as represented in the followingformulae (10), (11) and (12), using Rca and Rcb (temperature sensitiveresistors 13 a and 13 b) both connected to Rc (temperature sensitiveresistor 13);Rht×(Rc+Rca)=Ra×Rb  (10)Rht×(Rc+Rcb)=Ra×Rb  (11)Rht×(Rc+Rca+Rcb)=Ra×Rb  (12)

Combinations of the resistances represented by the formulae (9), (10),(11) and (12) can be realized by modifying connections betweenelectrodes 51, and these modifications can be inexpensively achievedbecause of no need of adjustment, such as trimming, using a laser, forexample, from the viewpoint of manufacturing process. Such a change ofthe bridge balance is effective particularly in absorbing a variation inthe heating temperature caused by a variation in the resistancetemperature coefficients.

Since the resistance temperature coefficients do not undergo a largevariation within the same lot, it is just required to modify connectionsbetween the electrodes 51. Thus, this embodiment basically requires noadjustment. The temperature sensitive resistors, such as represented byRca and Rcb shown in FIG. 11, for finely adjusting the heatingtemperature may be formed as serial resistances or parallel resistances.When there is a space left on the substrate 2, higher accuracy can beachieved by increasing the number of the temperature sensitive resistorsformed on the substrate.

Additionally, as understood from the formula (9), fine adjustment of theheating temperature can be realized regardless of the temperaturesensitive resistors for fine adjustment, described above, being combinedwith which one of the bridge resistances constituting the fixedtemperature difference bridge.

Other embodiments of the present invention will be described below.

The temperature characteristics shown in FIG. 4 have such flow ratedependency that there occurs a minus error at a low flow rate and a pluserror at a high flow rate. When the fixed temperature difference bridgeis entirely made up of the temperature sensitive resistors, a variationin temperature characteristics can be reduced, but it is difficult toeliminate such flow rate dependency.

To reduce the error caused by the flow rate dependency, as shown in FIG.11, a temperature sensor 8 is formed on the substrate 2 so that anoutput representing the temperature detected by the temperature sensor 8is obtained in addition to a flow rate output from a thermal type flowsensor 26. Then, as shown in FIG. 12, the flow rate output from thethermal type flow sensor 26 and the temperature sensor output areinputted to a compensation circuit 46. Errors of the temperaturecharacteristics can be further reduced by delivering the flow rateoutput after compensating for the flow rate dependency in thecompensation circuit 46.

The temperature sensor 8 is particularly preferably formed of the samematerial as that used for the fixed temperature difference bridge. Byforming the temperature sensor 8 in the same manufacturing process usingthe same material, the temperature sensor 8 is able to have the sameresistance temperature coefficient as that of each component of thefixed temperature difference bridge and the flow rate dependency can berelatively easily compensated.

Although the heat generating resistor 10 is formed in the thin wallportion 7 and is thermally insulated from the substrate 2, a slightamount of heat is conducted to the substrate 2, and the temperaturesensitive resistors, including 11, 12 and 13, also generates a slightamount of heat. Accordingly, the temperature of the substrate 2 rises ifthe substrate 2 has a relatively small area or has poor heat radiation.In that case, the temperature of the temperature sensor 8 also rises,thus resulting in a possibility that there occurs an error incompensation of the flow rate dependency. Taking into account such anerror, more effective compensation can be achieved by arranging thetemperature sensor 8 in a second thin wall portion 7 a to be thermallyinsulated from the substrate 2 as shown in FIG. 11.

As an alternative, the temperature sensor 8 may be incorporated in thecompensation circuit 46 although the error is slightly increased. Inthis case, the necessity of wiring for connection between thetemperature sensor 8 and the compensation circuit 46 is eliminated.

The temperature sensor 8 can also be used, as it is, to detect thetemperature of intake air, which is used for engine control. Inparticular, the structure wherein the temperature sensor 8 is arrangedin the second thin wall portion 7 a is more preferable because thatstructure provides a faster response.

One application example of the present invention will be described belowwith reference to FIG. 13.

By forming all the resistances of the fixed temperature differencebridge as the temperature sensitive resistors, as shown in FIG. 13, notonly the thermal type flow sensor 26, but also the differentialamplifier 41, the transistor 45, the compensation circuit 46, etc. canbe all formed on one substrate 2 made of the same semiconductor.

Even in attempting to realize an integral structure in the prior art,another circuit board (corresponding to 4) separate from the substrateis essential because of the presence of the fixed resistances for thefixed temperature difference bridge and the necessity of adjustment ofthe heating temperature. By employing the structure of the presentinvention in which all the resistances of the fixed temperaturedifference bridge are formed as the identical temperature sensitiveresistors, the circuit board 4 is not necessarily required in thepresent invention. As a result, the size and the production cost of aflow measuring apparatus can be further cut down.

A maximum advantageous obtained with the integration of the thermal typeflow sensor 26 and the circuit board 4 resides in that, when a sectionof the thermal type flow sensor 26 is cooled by intake air, thedifferential amplifier 41, the transistor 45, the compensation circuit46, etc., which are formed on the same substrate 2, are alsosimultaneously cooled, whereby the latter components always have thetemperature as that of the thermal type flow sensor 26.

In the prior art, when a temperature difference occurs between thetemperature of intake air and the circuit board (corresponding to 4) andthe temperature of the circuit board is not uniform, temperaturecharacteristics of electronic parts mounted on the circuit boardfluctuate in a complicated way, thus resulting in an error of the sensoroutput (corresponding to 44). Such an error can be greatly reduced withthe application of the present invention. The sub-passage 52 may have abent portion. The use of a bent-shaped sub-passage is effective inprotecting the thermal type flow sensor 26, including many resistancesand circuits formed therein, against dust, liquid droplets and backfirewhich are contained in the fluid flow.

Still another embodiment of the present invention will be describedbelow with reference to FIGS. 14, 15, 16 and 5.

FIG. 14 is a partial enlarged view of a thermal type flow sensor 26according to this embodiment, and FIGS. 15 and 16 are each a graphshowing an example of a temperature distribution measured along asection A—A in FIG. 14. FIG. 5 is a graph showing one example ofresistance changes of a heat generating resistor 10 when it is energizedand heated to about 250° C.

As shown in FIG. 14, the thermal type flow sensor 26 comprises a heatgenerating resistor 10 and the temperature sensitive resistors 11, 12and 13. Assuming here that the resistance value of the heat generatingresistor 10 under operation is Rht, the resistance value of thetemperature sensitive resistor 11 under operation is Ra, the resistancevalue of the temperature sensitive resistor 12 under operation is Rb,the resistance value of the temperature sensitive resistor 13 underoperation is Rc, and all the resistance temperature coefficients ofthese resistors have the same value α, a fixed temperature differencebridge expressed by the following formula (13), for example, can beformed:Rht×Rc=Ra×Rb  (13)

The resistance value of the heat generating resistor 10 always used in aheated state gradually changes as shown in FIG. 5. In FIG. 5, (X)represents resistance changes of a polysilicon thin-film resistor overtime, and (Y) represents resistance changes of a platinum thin-filmresistor over time. Thus, a tendency of resistance changes over timediffers depending on materials used for the heat generating resistor 10.

A description is first made of, with reference to FIG. 15, changes of atemperature distribution when the resistance value gradually increaseslike a polysilicon thin-film resistor. Because the bridge balancerepresented by the formula (13) is not changed even with an increase ofthe resistance value of the heat generating resistor 10, i.e., Rht,which is resulted from deterioration over time, the heating temperatureof the heat generating resistor 10 lowers in an amount corresponding tothe resistance increase of Rht.

FIG. 15 shows an actually measured example of such a lowering of theheating temperature. In FIG. 15, a curve (a) represents the temperaturedistribution before the resistance change, and a curve (b) representsthe temperature distribution after the resistance change. Comparing thecurves (a) and (b), it is understood that the changes of the temperaturedistribution are not even and the change amount is smaller in a portionat a higher temperature.

The reason resides in that the portion at a higher temperature undergoesa more significant change over time and exhibits a larger resistanceincrease, while it generates Joule heat in amount increasedcorresponding to the resistance increase as compared with the otherportion.

By positively utilizing the changes of the temperature distribution, itis possible to prevent a reduction of the heating temperature of theheat generating resistor 10 caused by resistance changes thereof overtime. More specifically, as shown in FIG. 14, Rb (temperature sensitiveresistor 12) is formed in the vicinity of an area in which thetemperature of the heat generating resistor 10 has a maximally highvalue, and Rc (temperature sensitive resistor 13) is formed in thevicinity of an area in which the temperature of the heat generatingresistor 10 has a relatively low value.

Although the heat generating resistor 10 is formed in the thin wallportion 7 and is thermally insulated from the substrate 2, a slightamount of heat is still conducted to the substrate 2. With theabove-mentioned pattern layout, therefore, Rb and Rc are subjected to aslight temperature difference between them.

When the temperature distribution is changed from one represented by (a)in FIG. 15 to another represented by (b) in FIG. 15 as a result of thetime-dependent change, the temperature in the Rb area is hardly changed,while the temperature in the Rc area slightly lowers. Accordingly, theresistance value Rc of the temperature sensitive resistor 13 is reducedand the bridge balance is changed, whereby a lowering of the heatingtemperature can be prevented. As an alternative, it is also possible tomore positively utilize the changes of the temperature distribution byarranging parts of the Rc and Rb patterns in the thin wall portion 7.

As a result of preventing changes of the heating temperature of the heatgenerating resistor 10 in such a manner, characteristic changes of theoutput 44 can also be prevented.

The concept of this embodiment is of course applicable to the thermaltype flow sensors, shown in FIGS. 1, 6 and 8, according to the otherembodiments of the present invention, and the same advantage isobtained.

A description is now made of, with reference to FIG. 16, changes of atemperature distribution when the resistance value gradually decreaseslike a platinum thin-film resistor. When the resistance value of theheat generating resistor 10 is reduced over time, the heatingtemperature of the heat generating resistor 10 rises contrary to theabove-described case of using a polysilicon thin-film resistor. FIG. 16shows an actually example of such a rise of the heating temperature. InFIG. 16, a curve (a) represents the temperature distribution before theresistance change, and a curve (c) represents the temperaturedistribution after the resistance change. Comparing the curves (a) and(c), it is understood that the changes of the temperature distributionare not even and the change amount is smaller in a portion at a highertemperature. The reason resides in that the portion at a highertemperature undergoes a more significant change over time and exhibits alarger resistance decrease, while it generates Joule heat reduced inamount corresponding to the resistance decrease as compared with theother portion.

The pattern arrangement of the temperature sensitive resistors in thiscase is also exactly the same as that in the above-described case. Morespecifically, as shown in FIG. 14, Rb (temperature sensitive resistor12) is formed in the vicinity of an area in which the temperature of theheat generating resistor 10 has a maximally high value, and Rc(temperature sensitive resistor 13) is formed in the vicinity of an areain which the temperature of the heat generating resistor 10 has arelatively low value. When the temperature distribution is changed as aresult of the time-dependent change, the temperature in the Rb area ishardly changed, while the temperature in the Rc area slightly rises.Accordingly, the resistance value Rc of the temperature sensitiveresistor 13 is increased, whereby a rise of the heating temperature canbe prevented.

Still another embodiment of the present invention will be describedbelow with reference to FIGS. 17 and 18.

FIG. 17 is a diagram showing a structure and a wiring pattern of athermal type flow sensor 26 according to this embodiment, and FIG. 18 isa circuit diagram including the thermal type flow sensor 26 shown inFIG. 17.

By connecting a temperature sensitive resistor 22 to an intermediateportion of a lead line extended from a heat generating resistor 10, asshown in FIG. 17, useless resistance between the heat generatingresistor 10 and its electrode 51 can be reduced. The smaller wiringresistance, the smaller a voltage of a power supply 42 is required toheat the heat generating resistor 10. In particular, a 12-V battery isemployed in automobiles, and a voltage of the battery is dropped toabout 6 V especially upon startup of an engine. For that reason, it ispreferable that the thermal type flow measuring apparatus 1 operates ata lower driving voltage.

Stated otherwise, in the structure of this embodiment, a led-outtemperature sensitive resistor 22 is formed midway one lead lineconnected to one input terminal of a differential amplifier 41. When theresistance value of the led-out temperature sensitive resistor 22changes depending on temperatures, the input voltage of the differentialamplifier 41 is changed and then appears as changes of the heatingtemperature of the heat generating resistor 10.

Corresponding to that structure, in the present invention, anothertemperature sensitive resistor 23 having a resistance value almost equalto that of the led-out temperature sensitive resistor 22 is formed inthe same manufacturing process midway the other lead line connected tothe other input terminal of the differential amplifier 41.

FIG. 18 shows a circuit diagram including the thermal type flow sensor26 of this embodiment. In the circuit configuration of this embodiment,the led-out temperature sensitive resistors 22, 23 are formed in therespective lead lines connected from the fixed temperature differencebridge to the input terminals of the differential amplifier 41. Byforming the led-out temperature sensitive resistors 22, 23 to have thesame resistance value and the same resistance temperature coefficient,however, the heating temperature is not changed. Consequently, withapplication of the present invention in the form of this embodiment, thepower supply voltage can be reduced without causing a variation intemperature characteristics.

A still another embodiment of the present invention will be describedbelow.

First, a supplementary description is made of the heating temperature ofthe heat generating resistor 10 with reference to FIGS. 23 (Table 1) and24 (Table 2). As shown in Table 2, the heating temperature of the heatgenerating resistor 10 is 170° C. when the ambient temperature is 20°C., and the temperature difference ΔT between the heating temperatureand the ambient temperature is 150° C. Likewise, when the ambienttemperature is 80° C., the temperature difference ΔT is 167° C. (=247°C.−80° C.).

In other words, the fixed temperature difference bridge has a tendencythat, as the ambient temperature rises, ΔT slightly increases. When theheat generating resistor 10 takes a higher temperature at the ambienttemperature which is relatively high, this implies that the heatgenerating resistor 10 has a smaller allowance for changes over time.

Some of related art is designed to be able to change ΔT of the heatgenerating resistor 10 as desired depending on changes of the ambienttemperature. In one known example, as shown in a circuit diagram of FIG.22, a fixed resistance 67 is connected in series to one temperaturesensitive resistor (corresponding to 11) of the fixed temperaturedifference bridge.

In view of the above, still another embodiment of the present inventionis intended to provide means for obtaining a similar advantage by usingthe temperature sensitive resistors.

Such an embodiment of the present invention will be described below withreference to FIGS. 19 and 20.

FIG. 19 shows a structure of a thermal type flow sensor 26 comprising aheat generating resistor 10 and temperature sensitive resistors 11, 12and 13, which are formed in the same manufacturing process using thesame material. When a polysilicon thin-film resistor is employed as thematerial, the resistance temperature coefficient is about 2000 ppm/° C.Then, second temperature sensitive resistors 24, 25 are formed of, e.g.,a platinum thin-film resistor having a different resistance temperaturecoefficient and are electrically connected to the temperature sensitiveresistor 13. The resistance temperature coefficient of the platinumthin-film resistor is about 3000 ppm/° C.

FIG. 20 is a circuit diagram including the thermal type flow sensor 26of this embodiment. With the above-mentioned structure shown in FIG. 20,a portion including a combination of the temperature sensitive resistor13 and the second temperature sensitive resistors 24, 25 exhibits largerresistance changes with respect to changes of the ambient temperaturethan the other temperature sensitive resistor 12.

By properly setting the resistance values of the second temperaturesensitive resistors 24, 25, therefore, ΔT of the heat generatingresistor 10 can be reduced as desired at a higher ambient temperature.

It is also possible to form electrodes 51 and the second temperaturesensitive resistors 24, 25 of the same material. In such a case, thisembodiment can be implemented with no need of a new additional step offorming the second temperature sensitive resistors 24, 25. A platinumthin film, an aluminum thin film or the like having a relatively highresistance temperature coefficient is suitable as the material for theelectrodes 51 and the second temperature sensitive resistors 24, 25.

As an alternative case, the heat generating resistor 10 and thetemperature sensitive resistors 11, 12 and 13 may have a higherresistance temperature coefficient than that of the second temperaturesensitive resistors 24, 25. In this case, a similar advantage can beobtained by combining the second temperature sensitive resistors 24, 25with the temperature sensitive resistor 11 or 12.

Further, as described above with reference to FIG. 5, a semiconductormaterial, such as a polysilicon thin film, and a metallic material, suchas a platinum thin film, show opposite tendencies in resistance changeover time. With application of the present invention in the form of thisembodiment, therefore, changes of the heating temperature of the heatgenerating resistor can be reduced by utilizing the fact that theresistance value of the heat generating resistor is increased, forexample, because of deterioration over time, while the resistance valuesof the second temperature sensitive resistors are reduced. As a result,this embodiment provides an advantage that a thermal type flow measuringapparatus 1 having higher reliability can be provided.

FIG. 21 shows an embodiment in which the present invention is applied toan internal combustion engine, in particular, a gasoline engine. In thisembodiment, the flow rate of intake air 101 supplied to the engine isdetected by the thermal type flow measuring apparatus 1 according to thepresent invention while the intake air flows through an intake passage,which is formed in an integral structure of an air cleaner 102, a body105, a duct 106, a throttle angle sensor 107, an idling air controlvalve 108 and a throttle body 109 with an intake manifold 110, orthrough a bypass. The detected signal is taken into a control unit 111in the form of voltage, frequency, etc. and is employed for control of astructure and subsystem of a combustion section constituted by aninjector 112, a tachometer 113, an engine cylinder 114, an exhaustmanifold 115, a gas 116, and an oxygen concentration meter 117.

The present invention is also applicable to a diesel engine because ithas substantially the same basic structure as that of a gasoline engine.More specifically, the thermal type flow measuring apparatus 1 of thepresent invention is disposed between an air cleaner (corresponding to102) and an intake manifold (corresponding to 115) of the diesel engineto detect the flow rate of intake air, and the detected signal is takeninto a control unit (corresponding to 111).

Recently, to be adapted for social demands such as more strictrestriction of automobile exhaust gas and protection against airpollution, studies have been intensively made on, for example, propanegas vehicles, natural gas vehicles, or vehicles in which a fuel cellusing hydrogen and oxygen, as fuel, to generate electric power and thevehicle is moved with a motor driven by the generated electric power.

The thermal type flow measuring apparatus of the present invention canbe likewise applied to a system for detecting the flow rate of a fluidand properly controlling the amount of supplied fuel in each of thosevehicles.

According to the embodiments described above, the flow rate of a fluidcan be detected with high accuracy even in a thermally severeenvironment such as represented by an engine room, and a thermal typeflow measuring apparatus having high reliability can be provided arelatively inexpensive cost.

1. A thermal type flow measuring apparatus comprising: a bridge circuithaving a first resistor, a second resistor, a third resistor connectedin series with said first resistor, and a fourth resistor connected inseries with said second resistor, wherein said first resistor is heatedby electric current supplied to said first resistor, said electriccurrent being controlled in order to maintain a temperature differencebetween said first resistor and said second resistor to a substantiallyconstant value, said first resistor, said second resistor, said thirdresistor, and said fourth resistor being formed on one side of a siliconsubstrate, being formed of a same material, and said first resistorbeing formed on a thin wall portion formed on said silicon substrate. 2.A thermal type flow measuring apparatus according to claim 1, whereinsaid first resistor, said second resistor, said third resistor, and saidfourth resistor are formed in a same manufacturing process.
 3. A thermaltype flow measuring apparatus according to claim 1, wherein said firstresistor, said second resistor, said third resistor, and said fourthresistor have a substantially same resistance temperature coefficient.4. A thermal type flow measuring apparatus according to claim 1, furthercomprising temperature detecting means disposed on both sides of saidfirst resistor.
 5. A thermal type flow measuring apparatus according toclaim 1, wherein said third resistor and said fourth resistor aredisposed close to each other.
 6. A thermal type flow measuring apparatusaccording to claim 1, wherein said third resistor and said fourthresistor are formed on said silicon substrate with an insulating layerdisposed therebetween.
 7. A thermal type flow measuring apparatusaccording to claim 1, further comprising a compensation circuit formedon said one side of said silicon substrate, said compensation circuitcompensating a signal from said bridge circuit.
 8. A thermal type flowmeasuring apparatus according to claim 1, further comprising a housingcovering said silicon substrate, said housing forming a fluid subpassage, wherein said silicon substrate is arranged on said fluid subpassage.
 9. A thermal type flow measuring apparatus according to claim8, wherein said silicon substrate and said fluid sub passage arearranged at an intake pipe of an engine.
 10. A thermal type flowmeasuring apparatus comprising: a bridge circuit having a firstresistor, a second resistor, a third resistor connected in series withsaid first resistor, and a fourth resistor connected in series with saidsecond resistor; and a fifth resistor, wherein said fifth resistor isheated by electric current supplied to said fifth resistor, heatgenerated from said fifth resistor being detected by said firstresistor, said electric current being controlled in order to maintain atemperature different between said first resistor and said secondresistor to a substantially constant value, said first resistor, saidsecond resistor, said third resistor, and said fourth resistor beingformed on one side of a silicon substrate, being formed of a samematerial, and said first resistor and said fifth resistor being formedon a thin wall portion formed on said silicon substrate.
 11. A thermaltype flow measuring apparatus according to claim 10, further comprisingtemperature detecting means disposed on both sides of said fifthresistor.
 12. A thermal type flow measuring apparatus according to claim10, wherein said first resistor, said second resistor, said thirdresistor, said fourth resistor, and said fifth resistor are formed in asame manufacturing process.
 13. A thermal type flow measuring apparatusaccording to claim 10, wherein said first resistor, said secondresistor, said third resistor, said fourth resistor and said fifthresistor have a substantially same resistance temperature coefficient.14. A thermal type flow measuring apparatus according to claim 10,wherein said first resistor, said second resistor, said third resistor,and said fourth resistor are formed in a same manufacturing process. 15.A thermal type flow measuring apparatus according to claim 10, whereinsaid first resistor, said second resistor, said third resistor, and saidfourth resistor have a substantially same resistance temperaturecoefficient.
 16. A thermal type flow measuring apparatus according toclaim 10, wherein said third resistor and said fourth resistor aredisposed close to each other.
 17. A thermal type flow measuringapparatus according to claim 10, wherein said third resistor and saidfourth resistor are formed on said silicon substrate with an insulatinglayer disposed therebetween.
 18. A thermal type flow measuring apparatusaccording to claim 10, further comprising a compensation circuit formedon said one side of said silicon substrate, said compensation circuitcompensating a signal from said bridge circuit.
 19. A thermal type flowmeasuring apparatus according to claim 10, further comprising a housingcovering said silicon substrate, said housing forming a fluid subpassage, wherein said silicon substrate is arranged on said fluid subpassage.